1. Field of the Invention
This present invention relates to a system, method, and associated apparatus for enabling the use of rasterstereographical principles for determining the curvature and surface detail across the surface of an object by using a computer analyzed rasterstereographic technique. More specifically, a projected light and dark pattern on the object is picked up by a video camera and the image is digitized by an image processor which calculates the surface detail by evaluating the distortion of the grid lines.
2. Description of the Prior Art
In recent years there has been increased interest in both qualitative and quantitative measurements of an object by topography. Particularly this increased interest has been in regard to corneal topography especially relating to keratorefractive procedures. Since keratorefractive procedures correct the refractive error of the eye by altering the curvature of the corneal surface, topographic measurements of the corneal curvature are important in planning, performing, and assessing the effect of these procedures.
Corneal topography has been proven of value for numerous uses including predicting the result of radial keralotomy, evaluating the design of epikeratophakia for myopia, diagnosis and staging of keratoconus, and guiding suture removal after corneal transplantation.
There have been previously reported photographic methods based on the keratoscopic disk system. (See "Corneal Topography," J. J. Rowsey, et al., Arch. Ophthalmol., Vol. 99, 1093 [1981]). This keratoscopic system consists of a series of black and white concentric rings on a circular disk. When this disk is placed in front of the eye, the rings are reflected by the corneal surface and their position, size, and spacing in the reflected image are determined by the corneal shape.
Current commercial systems utilizing illuminated concentric circular rings surrounding a viewing port through which photographs are taken have been known. If the cornea is spherical, the rings appear round and regularly spaced. If the cornea is oval or astigmatic, the rings are oval and the spacing varies in different axes. This is known as the placido disk technique.
These techniques, while providing a visual representation of the corneal surface, do not provide quantitative information. Computer programs have been developed which calculate the corneal profile and the optical power distribution on the corneal surface from placido disk images. See "Method for Calculation of Corneal Profile and Power Distribution," J. D. Ross, et al., Arch Ophthalmol., 1261 (1981).
Computer analyzing techniques have been developed for deriving quantitative information about the corneal shape from keratoscope photographs and displaying the results both numerically and graphically in easily understood forms. See "Computer-Assisted Corneal Topography, High Resolution Graphic Presentation and Analysis of Keratoscopy," S. D. Klyce, et al., Investigative Ophthalmology and Visual Science, Vol. 25, 1426 (1984).
Placido disk techniques for recording and quantifying the corneal surface have inherent limitations which reduce their clinical usefulness.
There are three main factors which limit the usefulness of the placido disk system. These factors are as follows: (1) The most central portion of the cornea is not imaged. This is due in part to the fact that there is a hole in the central portion of the placido disk through which the optical system for this technique views the cornea. This viewing port is devoid of any lighted spots or rings, and therefore there can be no reflected images on the cornea in this area. (2) The diameter of the placido disk determines how much of the corneal surface is covered by the reflected images. The smaller the diameter, the smaller the area of the cornea. The larger the diameter, the larger the area of the cornea that will be covered extending more toward the limbus or periphery of the cornea. (3) The distance between the cornea and the placido disk system also determines how much of the cornea is covered. The farther away the disk is from the cornea, the less the corneal coverage will be. The closer the disk is to the cornea, the greater the corneal coverage will be.
Other limitations of the placido disk techniques are that they do not extend to the corneal limbus due in part to shadows being cast from the eye lashes, brow and nose of the patient, nor do they work on corneas which do not have the necessary qualities to reflect an image of the disk due to conditions such as epithelial defects, scarring, or highly irregular shape.
Current computer methods being used to obtain quantitative measurements have been known to utilize photographic images acquired with the commercially available placido disk keratoscopes and are, therefore, subject to the same limitations discussed hereinbefore. In some such systems the data are entered into the computer by hand digitizing from these photographs, requiring a considerable amount of time, and the possible introduction of error during the digitization process.
While hand digitizing with some manually manipulated device is still being practiced, there is also known at least two systems for direct digitizing purposes, which systems have imaging cameras attached to the optics which, in turn, view through the central portion of the placido disk. These images are then taken directly into the computer for manipulation in calculating the corneal curvature and for determining the diopter powers.
These systems with direct digitization are still subject to the same problems as the placido disk systems having hand digitization. Although several attempts have been made to extend farther out into the limbus or periphery of the cornea, none of these systems have achieved this capability. These systems still inadequately handle corneas with very steep curvature or with a highly irregular surface.
It has been known to employ a rasterstereography method for measuring large body surfaces, curvature of the back, and reconstructive plastic surgery. Rasterstereography is an intermediate between stereography and moire topography and is a method of obtaining contour or topographic information where one of the cameras in a stereogrammetric pair is replaced with a light source which projects a grid of vertical parallel lines onto a subject.
One type of rasterstereographic system employs an electronic camera with a linear sensor, an x-y translator for image shifting, and a light source or projector. The camera and translator are connected to an on-line computer which produces an image scan of the large surface. See "Rasterstereographic Measurement and Curvature Analysis of the Body Surface," E. Hierholzer, et al., Biol. Photogr., Vol. 51, 11 (Jan. 1, 1983).
It has been known to employ a Rhonchi ruling in moire technique, which is normally a technique used for measuring the topography of a solid, nontransparent object. In moire topography a light source illuminates the Rhonchi ruling to cast shadows on the object to be measured. These shadows and the lines of Rhonchi ruling when viewed by either the eye or a camera interfere to produce contour lines of the object. See "Biostereometric Analysis in Plastic and Reconstructive Surgery," M. S. Karlan, et al., Plastic and Reconstructive Surgery, Vol. 62, (1978).
It has been known to attempt to determine corneal topography including moire techniques. A drawback is the low reflectivity of the cornea in that the cornea is a transparent, nondiffusing member, which does not allow for a good image of the grid to be formed on it.
It has been known to employ a microscope with a reticule referred to as a toposcope which uses the moire technique. A recticule is a grid or scale that is a standard piece of equipment in the moire technique. A series of straight parallel lines is imaged on the object. In the eyepiece of the microscope there is a reticule with the same number of lines. The two patterns interfere to produce the contours This instrument has been used to analyze contact lenses, but there is no evidence of using it to determine the contour of an eye. A drawback would be the low reflectivity of the cornea.
It has been known to use a fluorescein solution on a the eye, and a contact lens to determine the topography of a cornea. The fluorescein solution is placed on the eye followed by the placement of a contact lens. Blue-violet radiation produces a fluorescence pattern which gives an indication of the variable clearance between the known surface of the contact lens and the unknown cornea. For the measurements to be valid, the lens must be kept stationary, and in practice, diagnostic contact lenses are used to verify `K` readings in conjunction with refractive findings. See "Corneal Topography," T. W. Smith, M.D., Documenta Opthalmologica 43.2, pg. 262 (1977).
It has been known to determine corneal topography by stereographic techniques, in addition to holographic interferometric, and profile techniques See "Corneal Topography," pg. 263 cited in the preceding paragraph.
As the cornea is a transparent member which is nondiffusing to light, a grid projected onto the cornea is not visible unless a diffusing material is used to provide a surface on which an image can be visualized. It has been known to spray talcum powder on anesthetized corneas to obtain stereo photographs of the cornea.
Stereophotography is traditionally used to obtain the topography of a solid, nontransparent light diffusing object that has some texture. Stereophotography may utilize two cameras which view an object of interest from two different angles relative to a fixed center line between them. Stereophotography can also be accomplished by taking two sequential images with one camera. This is accomplished by focusing the camera on a fixed point and taking an exposure. The camera is then moved laterally a fixed distance, again focusing on the same point previously used in the first image and another exposure is made.
The two stereo photos are analyzed and one of the images is chosen as a reference image. Some object of interest is chosen and the displacement of the object in the opposite stereo image can be measured. From this displacement and the angle between the two shots, an elevation of an object can be calculated.
As the stereophotography method is used on solid objects, it has not been known to adequately obtain the topography of a cornea in that sufficient topographic detail of the cornea cannot be extracted.
It has been known to use an image processing system with a video camera, flash unit, and computer and display units in the field of opthalmology where the eye images are handled electronically. However, most of the study in the ophthalmology field has been in evaluating the optic nerve, retina, and corneal surface defects, and not for determining the curvature and related topographic details of the cornea. See "Development of An Imaging System for Ophthalmic Photography," J. W. Warnicki, et al., J. Biol. Photog. 53, 9 (1985).
In the holographic interferometric technique, it is known to use a beam splitter to direct the laser beam in one direction toward a camera and in the other direction toward an object. See "Corneal Topography," pg. 264 cited hereinbefore.
In spite of these known systems, methods, and instruments, there remains a very real and substantial need for a system, method, and device which more accurately and quickly determine quantitatively and qualitatively the contour of both a light diffusing, nontransparent object and a light nondiffusing, transparent object, such as a cornea.